This disclosure relates generally to a printing machine and a streak compensation method including scanning a P/R (Photoreceptor Belt) for image non-uniformities and controlling the printing process during run time to reduce or correct the image non-uniformities utilizing Spatially Varying Tone Reproduction Curves (STRCs). More particularly, streak profile measurements taken at a limited number of area coverage levels combined with a Printer Streaks Basis Function Model are used to estimate and project the streak behavior at all area coverage levels and at all inboard-to-outboard spatial locations to improve streak compensation transient response.
A typical electrophotographic, or xerographic, printing machine employs a photoreceptor, that is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoreceptor is exposed to a light image of an original document being reproduced. Exposure of the charged photoreceptor selectively dissipates the charge thereon in the irradiated areas to record an electrostatic latent image on the photoreceptor corresponding to the image contained within the original document. The location of the electrical charge forming the latent image is usually optically controlled. More specifically, in a digital xerographic system, the formation of the latent image is controlled by a raster output scanning device, usually a laser or LED source.
After the electrostatic latent image is recorded on the photoreceptor, the latent image is developed by bringing a developer material into contact therewith. Generally, the electrostatic latent image is developed with dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto. However, a liquid developer material may be used as well. The toner particles are attracted to the latent image, forming a visible powder image on the photoconductive surface. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to a sheet, such as paper or other substrate sheets, using pressure and heat to fuse the toner image to the sheet to form a print.
Electrophotographic printing machines of this type can produce color prints using a plurality of stations. Each station has a charging device for charging the photoconductive surface, an exposing device for selectively illuminating the charged portions of the photoconductive surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the respective electrostatic latent image. The images are developed, at least partially in superimposed registration with one another, to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a sheet. The transferred multi-color image is then permanently fused to the sheet forming the color print.
Although these xerographic printing machines usually produce a faithful reproduction of the original image, defects in the subsystems of the xerographic system may give rise to cross-process non-uniformities, commonly referred to as streaks or streak defects, which can be a significant factor affecting the overall image quality of the print. Streaks are primarily one-dimensional visible defects in the image that run parallel to the process direction, also referred to as the slow-scan direction. In a uniform gray level patch, streaks may appear as a variation in the reflectance, optical density, or in the colorimetric CIELAB L* value, among other units of color variation well known to practitioners of the art. As used herein, “gray” refers to the digital area coverage value of any single color separation layer, whether the toner is black, cyan, magenta, yellow, or some other color. In a color xerographic machine, streaks in single color separations that may be unobjectionable can cause an undesirable visible color shift for overlaid colors.
Conventional printing technologies contain several sources of streaks which cannot be satisfactorily controlled via printer design or printing system optimization. Streaks can be caused by “non-ideal” responses of xerographic components in the marking engine. The source of these artifacts is found in toner adhered on the wires, in dirt on the charging elements, P/R streaks, fuser originated streaks, charge contamination, etc. Streaks can also be caused by non-uniformity of the raster output scanning device spot-size or intensity variations. As shown in FIG. 1, a measured reflectance profile of a single color test image generated by the image forming machine is shown. The reflectance profile is generated by measuring the reflectivity of the image in the cross-process direction. The x-axis in FIG. 1 represents the pixel index in the cross-process direction. The pixel index value represents a location in the cross-process, or “fast scan”, direction. The measured reflectance profile illustrates streaks that would manifest themselves as undesired variations in cross-process L* in the test image measured on paper. A desired reflectance profile would be flat.
Various control schemes have been used for correcting streaks in image forming machines. ROS actuation for streak correction has been used, as disclosed US Publication No. 2006/0001911 A1 for “Closed-loop compensation of streaks by ROS intensity variation” by Viassolo et al. The intensity of the illumination source of the raster output scanner is controlled as a function of the fast-scan position to compensate for streaks.
Streak compensation techniques that measure current streak performance of the printer and adjust the digital image using Spatially Varying Tone Reproduction Curves (STRCs) to compensate for the streaks during run-time print jobs have been described in U.S. application Ser. No. 12/277,594, filed Nov. 26, 2008, incorporated by reference above. For high spatial frequency streaks, run time updates of the streak compensation actuator are useful to maintain or improve performance during long print runs. These updates occur every 1 to n belt revolutions, such as for example once every 6 belt revolutions, in typical image forming machines. However, it has been found that streaks can change rapidly due to rapidly changing states within the printer and it is desirable to increase the transient response of the run time control technique described therein to improve the response to new streaks or to streaks that were not completely compensated at cycle up.